By Martin R. Prince MD, PhD, Thomas M. Grist M.D., Jörg F. Debatin M.D., MBA (auth.)
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The acquisition of multiple signal averages is counterproductive. While concentrated paramagnetic contrast provides sufficient signal-to-noise to resolve vessels if the dose is sufficient, signal averaging would merely contribute toward a lengthening of the data collection time. Because of the importance ofbreath-holding, it is important to adjust the imaging time to the needs of each individual patient. Some patients with respiratory distress can barely hold their breath for 10 or 15 seconds, whereas other may easily suspend breathing for 30-40 seconds.
Another way to achieve some fat suppression without requiring extra time for extra pulses is to adjust the bandwidth and echo fraction to obtain an echo time where fat and water are out-of-phase. 5 ms is appropriate to maximize this chemical shift effect. What Is The Best Echo Time (TE)? The echo time should be short enough to eliminate dephasing artifacts and to minimize T2" signal decay. This requires an echo time less than about 3 ms. 5 ms), as mentioned above. Further reductions in echo time can shorten the TR and further reduce flow and susceptibility artifacts.
These allow repetition times that are substantially shorter than what is achieved with their standard 3D gradient echo pulse sequence. These optimized pulse sequences with the shortest possible repetition time often make it possible to complete the data acquisition within a breath-hold, but they may not be as useful for applications outside of MR angiography because of the tendency for more aliasing (wraparound artifact) in the slice direction. What Is The Best Flip Angle? Gradient echo imaging requires selection of the flip angle (sometimes known as tip angle).